Distributed-constant dispersive network

ABSTRACT

A microwave dispersive network for wideband signal processing systems is disclosed in which a plurality of all-pass, quarterwave, coupled transmission lines constructed in dielectric stripline are connected in cascade to synthesize a desired dispersive characteristic.

United States Patent Herring June 6, 1972 [54] DISTRIBUTED-CONSTANT DISPERSIVE NETWORK [72] Inventor: Frederick G. Herring, Wantagh, NY.

[73] Assignee: I-Iazeltine Corporation [22] Filed: May 27, 1970 [21] App1.No.: 41,031

[52] U.S. Cl. ..333/30 R, 333/73 S, 333/84 M [51] ..H0lp l/l8, H03h 7/28 [58] Field of Search ..333/73 C, 73 S, 84 M, 10, 3O

;56] References Cited UNITED STATES PATENTS 2,760,169 8/1956 Englemann ..333/73 2,964,718 12/1960 Paokard ..333/73 S 3,104,362 9/1963 Matthei ..333/73 W 2,945,519 7/1960 Matthei ..333/73 W 2,984,802 5/1961 Dyer et al. ....333/73 S 2,859,417 11/1958 Arditi ....33/73 W 2,968,012 1/1961 Allstadter ..33/73 S OTHER PUBLICATIONS IRE Transactions on Circuit Theory, June 1958, p. 104- 1 13 Primary Examiner-Herman Karl Saalbach Assistant Examiner-C. Baraff AttorneyEdward A. Onders [57] ABSTRACT A microwave dispersive network for wideband signal processing systems is disclosed in which a plurality of all-pass, quarter-wave, coupled transmission lines constructed in dielectric stripline are connected in cascade to synthesize a desired dispersive characteristic.

13 Claims, 6 Drawing Figures EATENTEDJUH 6 I972 SHEET 1 OF 2 FIG.

FIG. lc

FIG. 2

PATENTEDJUH s 1912 SHEET 2 BF 2 3fo FREQUENCY FIG. 3

fc FREQUENCY wo wEE.

FIG. 4

DISTRIBUTED-CONSTANT DISPERSIVE NETWORK BACKGROUND OF THE INVENTION This invention relates to microwave dispersive networks, and in particular to low distortion, distributed-constant, dispersive networks for use in wideband signal processing systems.

Microwave dispersive networks are highly important in modern wideband signal processing systems to implement bandwidth and time expanders and compressors, wideband instantaneous spectrum analyzers, compressive receivers, electronically variable delays, and antenna beam steering. In radar systems, dispersive networks implement pulse compression to improve resolution in range and velocity, and improve dis crimination against noise.

Several different types of distributed-constant, microwave wideband dispersive networks have been used to attempt to provide high bandwidth-time products and very wide bandwidths, specifically: the four port hybrid ring with two of the ports terminated in dual reactive impedances; the meander line; and the waveguide operated near cutoff. What has not been appreciated in the prior art, however, is the fact that to implement high bandwidth-time products and very wide bandwidths an all-pass network must be used. The four port hybrid ring network is a limited approximation to an all-pass network, since it is exactly all-pass at only one frequency. The approximation is good only over a small fractional bandwidth, and thus the networks usefulness is limited to relatively small bandwidths and to systems which have a low bandwidth-time product. The meander line is also an approximation to an allpass network and this approximation results in a high VSWR which causes distortion in the phase and amplitude characteristics of the network and mismatch losses. The waveguide near cutoff is inherently not all-pass since it must be operated near cutoff to obtain a dispersion, and amplitude distortion is, therefore, unavoidable.

SUMMARY OF THE INVENTION It is therefore an object of this invention to provide a distributed-constant, microwave dispersive network which is allpass and suitable for implementing high bandwidth-time products and very wide bandwidths in wideband microwave signal processing systems.

In accordance with the present invention there is provided a dispersive network which includes a plurality of pairs of first conductive strip members. The members of each pair are parallel to each other and parallel to the members of all other pairs and are separated by a dielectric medium. Each of the pairs has a predetermined length and has other dimensions which are substantially the same as those of all of the other pairs 56 that all of the pairs have substantially the same characteristic impedance. Also included are a plurality of electrical interconnection means each for interconnecting a pair of first strip members at a selected point along their length to form a corresponding plurality of all-pass, coupled transmission lines. Each of the transmission lines has a length that is resonant at a selected frequency and the plurality of transmission lines includes lines of different lengths to provide the network with a desired dispersive characteristic for selected different frequencies. Further included are means, having substantially the same characteristic impedances as the transmission lines and positioned coplanar with the conductive strip members, for connecting the transmission lines in cascade. Finally, included is a first planar conductive member positioned adjacent to one of the sides of the cascade of coupled transmission lines and separated from the transmission lines by a dielectric medium having the same dielectric constant as that separating the first conductive stnp members, thereby serving as a ground plane for the coupled transmission lines.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description, taken in connection with the accompanying drawings, while its scope will be pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. la is an enlarged, cross-sectional end view of one of the transmission lines of the dispersive network of F IG. lb;

FIG. lb is a top plan view of one embodiment of a microwave dispersive network constructed in dielectric stripline and embodying the invention, showing the coupled transmission lines of FIG. la in cascade;

FIG. 10 is a cross-sectional end view showing the interconnection members which connect in cascade the transmission lines of the network of FIG. 1b;

FIG. 2 is a schematic representation of an all-pass, coupled transmission line of FIGS. la, lb, and 1c;

FIG. 3 is a graphical illustration of the periodic time-delay characteristic of an all-pass coupled transmission line; and

FIG. 4 is a graphical illustration of one type of dispersive characteristic which may be synthesized by a microwave dispersive network constructed in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION:

One embodiment of a microwave dispersive network utilizing a cascade of all-pass, quarter-wave coupled transmission lines to synthesize a desired dispersive characteristic is illustrated in FIGS. 1a, 1b, and 1c. The dispersive network illustrated is constructed in dielectric stripline and includes a planar dielectric member such as thin dielectric sheet 10, and a plurality of pairs of first conductive strip members illustrated as strips of conductive material 11. The conductive strips 11 of each pair are affixed to opposite sides of dielectric sheet 10 in register and parallel to all other conductive strips affixed to the same side of the dielectric sheet. The members of a pair have a predetermined length 12, the transmission line length, related to the desired dispersive characteristic of the network, and other dimensions which are substantially the same as those of all other pairs.

The physical dimensions of the first strip members 11 are given in FIG. Ia. They are: w, the conductive strip width; and t, the cross-sectional thickness of the conductive strips. These dimensions are identical for all of the first strip members to ensure that the characteristic impedance of each transmission line is the same. The transmission line dimensions determine the electrical impedance and coupling characteristics of the lines, and the relationship between the two is well known as is shown by such publications as S. B. Cohn, Characteristic Impedance of Broadside Coupled Strip Transmission Lines," IRE Transactions on Microwave Theory and Techniques, Vol. M'IT8, Nov. 1960; and Thickness Corrections for Capacitive Obstacles and Strip Conductors, IRE Transactions on Microwave Theory and Techniques, Vol. MTT-8, November 1960.

A plurality of electrical interconnection means, such as the second strips of conductive material 13 shown in FIG. 1a, are each affixed to a pair of the first conductive strips 11. Each of the strips 13 interconnects a corresponding pair of the first conductive strips 11 at a selected point along their length through dielectric sheet 10 to form an all-pass, coupled transmission line of each pair, with each transmission line thus formed having substantially the same characteristic impedance. In this manner, the length 12 of each transmission line is made equal to the quarter wavelength of a corresponding predetermined frequency lying within the selected bandwidth of the dispersive characteristic to be synthesized. A cascade of transmission lines of different lengths imparts to the network the desired dispersive characteristic for frequencies in the selected band. Although the described embodiment used the quarter-wave frequency time delays of each transmission line to synthesize the desired dispersive characteristic, it is not restricted to the use of quarter-wave lines, and any odd integer multiple of the quarter-wavelength may be used as the length of the transmission lines.

The dispersive network also includes means for connecting the transmission lines in cascade, illustrated as third conductive strips 14 in FIG. lb which are positioned coplanar with the first conductive strip members 11 as shown in FIG. 1c. The strips 14 have substantially the same characteristic impedance as the coupled quarter-wave transmission lines and interconnect selected unconnected ends of the transmission lines alternately on either side of the dielectric sheet so as to form a continuous cascade of coupled transmission lines as shown in FIG. lb. If desired, the entire cascade of lines can be realized on a single double-sided printed circuit board by etching the first and third conductive strips from conductive material affixed to the board, and then interconnecting pairs of the first strips through the board with second conductive strips.

The network further includes a pair of planar conductive members such as thin sheets of conductive material 15 and 16, which serve as ground planes for the cascade of transmission lines. Each of the planar conductive members is positioned adjacent a corresponding side of the dielectric sheet 10, and are separated from each other by a predetermined distance b, the ground plane spacing, shown in FIG. la. The spaces between the dielectric sheet and planar conductive sheets and 16 are filled with dielectric material, such as the dielectric sheets 17 and 18 of FIGS. la and 10, having the same dielectric constant as dielectric sheet 10.

The first conductive strip members 11 may be positioned coplanar with each other, or positioned opposite to each other in different planes as shown in FIGS. 1a and lb, in register or off-set. The dual-plane configuration wherein the stripmembers 11 are positioned opposite to each other in register as shown in FIG. 1a is preferred since maximum electrical coupling between the members of each pair, and therefore maximum delay, can be obtained in this configuration. In the coplanar configuration, either one or two ground planes may be used, the transmission line being positioned between the two ground planes if the second plane is used. In the dualplane configuration two ground planes are generally necessary for proper operation. The invention, however, is in no way limited to these specific configurations and it is not intended here to do so. In either configuration, the dielectric medium used to separate the first conductive strip members 11, and separate the ground plane or planes from the transmission lines must be homogeneous throughout, i.e., have the same dielectric constant. I

The thin dielectric sheet 10 of FIGS. 1a, lb, and 1c separating conductive strip members 11 may be any dielectric medium, including air. In the latter case, the conductive strip memhers I 1 would be positioned in the dual plane or coplanar configuration, as desired, and supported and separated by rigid dielectric spacers. However, it is preferred to affix the members to a planar dielectric member such as dielectric sheet 10 in order to simplify construction. In this configuration a cascade of transmission lines can be produced simply by etching the members on opposite sides of a printed circuit board, which serves as the dielectric sheet. However, the dielectric medium used to separate the strip members 1 1 must have substantially the same dielectric constant as the dielectric medium used to separate the transmission lines from the ground planes.

As stated previously, the first conductive strip members are capacitively coupled together. As the coupled transmission line spacing s, shown in FIG. 4, is decreased, the coupling between the lines will increase. This results in an increase in the maximum time delay and in network efficiency. To prevent undesired coupling between adjacent transmission lines, the first conductive strips 11 on each side of the thin dielectric sheet 10 are separated from all other first conductive strips 11 on the same side of the dielectric sheet 10 by a distance 19 which is greater than the ground plane spacing b separating the pair of thin conductive sheets 15 and 16.

Dissipative losses occur in the coupled transmission lines and in the dielectric sheets 17 and 18, and are primarily controlled by the ground spacing b. The larger the ground spacing, the lower will be the peak and differential losses in the network. To reduce these losses further, the dispersive characteristic is usually synthesized with the maximum time delay at the lowest frequency. Also, since the time delay of the cascade is equal to the sum of the time delays of the individual transmission lines, the transmission lines may be arranged in any order of length.

In order to more clearly illustrate and explain the operation of the all-pass, coupled transmission lines of the FIGS. la, 1b, and 1c embodiment reference is now had to FIG. 2 of the accompanying drawings, which illustrates a schematic representation of such a transmission line. The transmission line of FIG. 2 includes a pair of first conductive members 20 which are positioned parallel to each other, above a ground plane 21 and interconnected at a selected point along their lengths 22 by an interconnecting strip 23 so that the transmission line has a predetermined length related to the desired dispersive characteristic to be synthesized. The members 20 are separated from each other by a dielectric medium and thus capacitively coupled together to provide a dispersive time delay characteristic. The members are also separated from the ground plane by a dielectric medium having the same dielectric constant as the medium separating members 20. The allpass, coupled transmission line formed by the interconnected members is driven and loaded with respect to the ground plane at input terminal 24 and output terminal 25, respectively.

FIG. 3 shows the periodic dispersive time-delay characteristic of an all-pass, coupled transmission line such as that of FIG. 2. The maximum time delay for a transmission line of any given length occurs at odd multiples of the frequency where the quarter-wavelength, i.e., one quarter of the wavelength, at the frequency, is equal to the length 12 of the transmission line. Thus, the maximum time delay for a particular frequency is obtained by constructing the all-pass, coupled transmission line length to be one quarter of the wavelength of the particular frequency.

As stated previously, each pair of first conductive strip members 11 which form an all-pass transmission line are capacitively coupled together via a dielectric medium. This coupling is important since it is this coupling between the members which causes the time delay characteristic of the transmission line to be dispersive. Thus, if the capacitive coupling between the members 11 were increased, the peaks of the time delay curve illustrated in FIG. 3 would increase.

A plurality of such all-pass coupled transmission lines may be connected in cascade as shown in FIG. lb to synthesize any desired dispersive characteristic; the linear characteristic illustrated in FIG. 4 being one example. A desired dispersive characteristic is synthesized by using the quarter-wave frequency delay contribution of each of the cascaded transmission lines as shown in FIG. 4. Since each of the transmission lines of the cascade is all-pass and of a different length, the time delay of the network formed by the cascade is equal to the sum of the time delays of the individual coupled transmission lines. However, since the time delay curve of each transmission line is periodic, the dispersive characteristic should be synthesized in the frequency band between f the quarter-wave frequency of the longest line in the cascade, and approximately 2f the frequency at which the eriodic delay curve reaches a minimum between the delay contributions at f and 3f This avoids delay contributions from the remainder of the periodic time delay curves of the transmission lines in the cascade. The superimposed delay curves for the transmission lines of such a cascade are shown in FIG. 4. Generally, the quarter-wave frequency delay contribution of each transmission line is used to synthesize a dispersive characteristic. However, the characteristic may be synthesized using the delay contribution occurring at any odd integer multiple of the quarter-wave frequency of each transmission line in the cascade. This technique is especially advantageous when synthesizing a dispersive characteristic which has a small fractional bandwidth since the network efficiency, i.e., the bandwidth-time product divided by the number of transmission lines required to synthesize a given dispersive characteristic, is increased substantially.

In order to synthesize a desired dispersive characteristic, a suitable distribution of quarter-wave frequency delay contributions within the selected bandwidth must be determined. This is substantially the same as synthesizing a dispersive characteristic using lumped-constant bridged-tee networks, for which design procedures are well known. These procedures may be found in T.R.OMeara, The Synthesis of Band-Pass, All-Pass Time Delay Networks with Graphical Approximation Techniques, Hughes Aircraft C0., Research Laboratories, Malibu, California, Report No. l 14, Feb. 1962; and RS. Brandon, The design Methods for Lumped-Constant Dispersive Networks Suitable for Pulse Compression Radar, Marconi Review, Vol. XXVll, No. 159, 1965. These procedures, used in conjunction with the equation given for the time delay of an all-pass, coupled transmission line in W.J.D. Steenaart, The Synthesis of Coupled Transmission Line All-Pass Networks in Cascades of l to n," IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-l 1, Jan. 1963, provide the necessary information to calculate the quarter-wave transmission line lengths in order to synthesize a dispersive network in accordance with the present invention.

For example, the first step in synthesizing a linear dispersive characteristic, such as that of FIG. 4, for a desired bandwidthtime product and bandwidth is to determine the quarter-wave frequencies for the best possible match to the desired dispersive characteristic, by using the procedures disclosed in the previously mentioned literature of Brandon, OMeara, and Steenaart. The first conductive strip members 11, the dimensions of which are determined by the techniques disclosed in Cohn, are then affixed to a dielectric sheet. A slit is cut through each pair of first conductive members 1 l and through the dielectric sheet at a point such that the length of the transmission line up to the point of the slit, is equal to onequarter of the wavelength of a corresponding one of the selected quarter-wave frequencies. A second conductive strip member 13 is placed in each slit and affixed, such as by soldering, to the first strip members 11 to form a quarter-wave transmission line. The separate transmission lines are then cascaded by attaching third conductive strips 14 to selected alternate pairs of unconnected ends of the strip members 11 on each side of dielectric sheet 10 as shown in FIGS. 1b and 1c.

It will be noted that in the illustrated embodiment a tab extension 26 is formed beyond the point of interconnection of the first members 11 and second conductive strip members 13. This extension provides a correction for a discontinuity created by the interconnection. The optimum extension length must be determined empirically to obtain a maximum VSWR for a given bandwidth, and it has been found that it is solely a function of the ground plane spacing b and the coupled transmission line spacing s.

The ground planes 15 and 16 are then added and separated by the required spacing b. The ground planes are spaced from the dielectric sheet 10 by additional sheets of dielectric material 17 and 18. As stated previously, the dielectric material used throughout must be homogeneous, i.e., have the same dielectric constant.

Several dispersive network subsections, having less than the desired bandwidth-time product, can be synthesized in the above manner and then these identical subsections can be cascaded to implement the desired bandwidth-time product. Since it is usually desired that the bandwidth-time product of a network be large, it is preferable to synthesize network subsections with lower bandwidth-time products, retaining their required bandwidth, and to achieve the total dispersion by cascading the subsections. The bandwidth-time products of these subsections are additive when cascaded.

A typical dispersive network of the type herein described may have the following parameters:

Dielectric Sheet 10 .0023 inch thick printed circuit board First Conductive Strip Members 1 l 1 02. copper strips 0.0015 inch thick and 0.278 inch wide Third Conductive Strip Members 14 copper strips 0.816 inch wide and 0.0015 inch thick Dielectric Sheets & Ground Plane /4 inch thick sheets of Tellite 3B material each backed on 1 side by copper Ground Plane Spacing b 0.25 inch 1,000 larger These parameters result in a transmission line impedance of 30 ohms, a bandwidth of 500 MHz, and a center frequency of 1000 MHz. The network consisted of 20 transmission lines, had a bandwidth-time product of 10, and a dispersive characteristic of the linear type illustrated in FIG. 4, having a fractional bandwidth equal to 0.5, and a dispersion equal to 20 nsec. Ten of the above networks when cascaded would provide a larger network with a bandwidth-time product equal to 100.

While there have been described what are at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of this invention.

What is claimed is:

1. A dispersive network, comprising:

a plurality of pairs of first conductive strip members with the members of each pair being parallel to each other and parallel to the members of all other pairs and being separated by a dielectric medium, each pair having a predetermined length and having other dimensions which are substantially the same as those of all other of said pairs so that all said pairs have substantially the same characteristic impedance;

a plurality of electrical interconnection means, each for interconnecting a pair of said first strip members at a selected point along their length to form a corresponding plurality of all-pass, coupled transmission lines each of which has a length that is resonant at a selected frequency and said plurality including transmission lines of different lengths to provide said network with a desired dispersive characteristic for selected different frequencies;

means, having substantially the same characteristic impedance as said transmission lines, positioned coplanar with said conductive strip members for connecting said transmission lines in cascade;

and a first planar conductive member positioned adjacent to one of the sides of the cascade of coupled transmission lines and separated from said transmission lines by a dielectric medium having the same dielectric constant as that separating said first conductive strip members, thereby serving as a ground plane for said coupled transmission lines.

2. A dispersive network as recited in claim 1, further comprising a second planar conductive member positioned adiacent to the other side of said cascade of coupled transmission lines and separated from said transmission lines by the same dielectric medium as that separating said first conductive strip members, thereby providing a pair of ground planes for said coupled transmission lines.

3. A dispersive network as recited in claim 2, wherein each of said interconnecting means interconnects a corresponding pair of said first strip members at such a point along their length that the transmission line formed thereby has a length equal to an odd integer multiple of one quarter of the wavelength of a predetermined frequency lying within a selected band of frequencies, thereby imparting to said network a desired dispersive characteristic for frequencies in said selected band.

4. A dispersive network as recited in claim 3, wherein each of said pairs of first conductive strip members is separated from all other pairs of members by a distance which is greater than the distance separating said pair of planar conductive members to prevent undesired coupling between adjacent transmission lines.

5. A dispersive network as recited in claim 4, wherein the members of each pair of said first members are coplanar with each other and the members of all other pairs.

6. A dispersive network comprising:

a plurality of pairs of first conductive strip members with the members of each pair being positioned opposite to each other in different parallel planes and parallel to all other first members in the same plane and being separated by a dielectric medium, each pair having a predetermined length and having other dimensions which are substantially the same as those of all other pairs so that all said pairs have substantially the same characteristic impedance;

a plurality of electrical interconnection means, each for interconnecting a pair of said first strip members at a selected point along their length to form a corresponding plurality of all-pass, coupled transmission lines each of which has a length that is resonant at a selected frequency and said plurality including transmission lines of different lengths to provide said network with a desired dispersive characteristic for selected different frequencies;

means, having substantially the same characteristic impedance as said transmission lines and positioned coplanar with said conductive strip members, for connecting said transmission lines in cascade;

and a pair of planar conductive members, each positioned adjacent a corresponding one of the sides of the cascade of coupled transmission lines and separated from said first conductive strip members by a dielectric medium having the same dielectric constant as that which separates said first members, thereby providing a pair of ground planes for said cascaded transmission lines.

7. A dispersive network as recited in claim 6, wherein each of said interconnecting means interconnects a corresponding pair of said first strip members at such a point along their length that the transmission line formed thereby has a length equal to an odd integer multiple of one quarter of the wavelength of a predetermined frequency lying within a selected band of frequencies, thereby imparting to said network a desired dispersive characteristic for frequencies in said selected band.

8. A dispersive network as recited in claim 7, wherein each of said pairs of first conductive strip members is separated from all other pairs of members by a distance which is greater than the distance separating said pair of planar conductive members to prevent undesired coupling between adjacent transmission lines.

9. A dispersive network as recited in claim 8, wherein the dielectric medium separating said first conduc-tive members is a planar dielectric member having said first conductive strip members of each pair affixed to opposite planar sides of said dielectric member in register and parallel to all other first members affixed to the same side of said dielectric member.

10. A dispersive network constructed in dielectric strip line using a cascade of all-pass quarter-wave coupled transmission lines, comprising:

a first thin sheet of dielectric material;

a plurality of pairs of first strips of conductive material with the conductive strips of each pair being affixed to opposite sides of said first dielectric sheet in register and parallel to all other first conductive strips afi'rxed to the same side of said first dielectric sheet, each pair having a predetermined length and having other dimensions which are substantially the same as those of all other of said pairs so that all said pairs have substantially the same characteristic impedance;

a plurality of second strips of conductive material, each affixed to a pair of said first conductive strips through said first dielectric sheet for interconnecting said pair of first conductive strips at a selected point along their length to form a correspondiqg plurality of all-pass quarter-wave transmission lines o differing lengths with each of said transmission lines resonant at a selected frequency to provide said network with a desired dispersive characteristic for selected different frequencies;

a plurality of third strips of conductive material, having substantially the same characteristic impedance as said transmission lines, affixed to the sides of said first dielectric sheet and connected between selected ends of said first conductive strips for coupling said separate transmission lines in cascade;

and a pair of thin sheets of conductive material, each positioned adjacent a corresponding one of the sides of said first dielectric sheet, separated from each other by a predetermined distance and from said first dielectric sheet by second sheets of dielectric material having the same dielectric constant as said first dielectric sheets, thereby serving as ground planes for said cascaded transmission lines.

11. A dispersive network as recited in claim 10, wherein each of said second strips interconnects a corresponding pair of said first strip members at such a point along their length that the transmission line formed thereby has a length equal to one quarter of the wavelength of a predetermined frequency lying within a selected band of frequencies, thereby imparting to said network a desired dispersive characteristic for frequencies in said selected band.

12. A dispersive network as recited in claim 11, wherein said first conductive strips on each side of said first dielectric sheet are separated from all other first conductive strips on the same side of said first dielectric sheet by a distance which is greater than the distance separating said pair of thin sheets of conductive material to prevent undesired coupling between adjacent transmission lines.

13. A dispersive network as recited in claim 12, in which said first conductive strips and said third conductive strips are circuit conductor patterns etched from conductive material affixed to said first dielectric sheet. 

1. A dispersive network, comprising: a plurality of pairs of first conductive strip members with the members of each pair being parallel to each other and parallel to the members of all other pairs and being separated by a diElectric medium, each pair having a predetermined length and having other dimensions which are substantially the same as those of all other of said pairs so that all said pairs have substantially the same characteristic impedance; a plurality of electrical interconnection means, each for interconnecting a pair of said first strip members at a selected point along their length to form a corresponding plurality of all-pass, coupled transmission lines each of which has a length that is resonant at a selected frequency and said plurality including transmission lines of different lengths to provide said network with a desired dispersive characteristic for selected different frequencies; means, having substantially the same characteristic impedance as said transmission lines, positioned coplanar with said conductive strip members for connecting said transmission lines in cascade; and a first planar conductive member positioned adjacent to one of the sides of the cascade of coupled transmission lines and separated from said transmission lines by a dielectric medium having the same dielectric constant as that separating said first conductive strip members, thereby serving as a ground plane for said coupled transmission lines.
 2. A dispersive network as recited in claim 1, further comprising a second planar conductive member positioned adjacent to the other side of said cascade of coupled transmission lines and separated from said transmission lines by the same dielectric medium as that separating said first conductive strip members, thereby providing a pair of ground planes for said coupled transmission lines.
 3. A dispersive network as recited in claim 2, wherein each of said interconnecting means interconnects a corresponding pair of said first strip members at such a point along their length that the transmission line formed thereby has a length equal to an odd integer multiple of one quarter of the wavelength of a predetermined frequency lying within a selected band of frequencies, thereby imparting to said network a desired dispersive characteristic for frequencies in said selected band.
 4. A dispersive network as recited in claim 3, wherein each of said pairs of first conductive strip members is separated from all other pairs of members by a distance which is greater than the distance separating said pair of planar conductive members to prevent undesired coupling between adjacent transmission lines.
 5. A dispersive network as recited in claim 4, wherein the members of each pair of said first members are coplanar with each other and the members of all other pairs.
 6. A dispersive network comprising: a plurality of pairs of first conductive strip members with the members of each pair being positioned opposite to each other in different parallel planes and parallel to all other first members in the same plane and being separated by a dielectric medium, each pair having a predetermined length and having other dimensions which are substantially the same as those of all other pairs so that all said pairs have substantially the same characteristic impedance; a plurality of electrical interconnection means, each for interconnecting a pair of said first strip members at a selected point along their length to form a corresponding plurality of all-pass, coupled transmission lines each of which has a length that is resonant at a selected frequency and said plurality including transmission lines of different lengths to provide said network with a desired dispersive characteristic for selected different frequencies; means, having substantially the same characteristic impedance as said transmission lines and positioned coplanar with said conductive strip members, for connecting said transmission lines in cascade; and a pair of planar conductive members, each positioned adjacent a corresponding one of the sides of the cascade of coupled transmission lines and separated from said first conductive strip members by a dielectric medium having the same diElectric constant as that which separates said first members, thereby providing a pair of ground planes for said cascaded transmission lines.
 7. A dispersive network as recited in claim 6, wherein each of said interconnecting means interconnects a corresponding pair of said first strip members at such a point along their length that the transmission line formed thereby has a length equal to an odd integer multiple of one quarter of the wavelength of a predetermined frequency lying within a selected band of frequencies, thereby imparting to said network a desired dispersive characteristic for frequencies in said selected band.
 8. A dispersive network as recited in claim 7, wherein each of said pairs of first conductive strip members is separated from all other pairs of members by a distance which is greater than the distance separating said pair of planar conductive members to prevent undesired coupling between adjacent transmission lines.
 9. A dispersive network as recited in claim 8, wherein the dielectric medium separating said first conduc-tive members is a planar dielectric member having said first conductive strip members of each pair affixed to opposite planar sides of said dielectric member in register and parallel to all other first members affixed to the same side of said dielectric member.
 10. A dispersive network constructed in dielectric strip line using a cascade of all-pass quarter-wave coupled transmission lines, comprising: a first thin sheet of dielectric material; a plurality of pairs of first strips of conductive material with the conductive strips of each pair being affixed to opposite sides of said first dielectric sheet in register and parallel to all other first conductive strips affixed to the same side of said first dielectric sheet, each pair having a predetermined length and having other dimensions which are substantially the same as those of all other of said pairs so that all said pairs have substantially the same characteristic impedance; a plurality of second strips of conductive material, each affixed to a pair of said first conductive strips through said first dielectric sheet for interconnecting said pair of first conductive strips at a selected point along their length to form a corresponding plurality of all-pass, quarter-wave transmission lines of differing lengths with each of said transmission lines resonant at a selected frequency to provide said network with a desired dispersive characteristic for selected different frequencies; a plurality of third strips of conductive material, having substantially the same characteristic impedance as said transmission lines, affixed to the sides of said first dielectric sheet and connected between selected ends of said first conductive strips for coupling said separate transmission lines in cascade; and a pair of thin sheets of conductive material, each positioned adjacent a corresponding one of the sides of said first dielectric sheet, separated from each other by a predetermined distance and from said first dielectric sheet by second sheets of dielectric material having the same dielectric constant as said first dielectric sheets, thereby serving as ground planes for said cascaded transmission lines.
 11. A dispersive network as recited in claim 10, wherein each of said second strips interconnects a corresponding pair of said first strip members at such a point along their length that the transmission line formed thereby has a length equal to one quarter of the wavelength of a predetermined frequency lying within a selected band of frequencies, thereby imparting to said network a desired dispersive characteristic for frequencies in said selected band.
 12. A dispersive network as recited in claim 11, wherein said first conductive strips on each side of said first dielectric sheet are separated from all other first conductive strips on the same side of said first dielectric sheet by a distance which is greater than the distance separating said pair of thin sheets Of conductive material to prevent undesired coupling between adjacent transmission lines.
 13. A dispersive network as recited in claim 12, in which said first conductive strips and said third conductive strips are circuit conductor patterns etched from conductive material affixed to said first dielectric sheet. 